Mach-Zehnder interferometers (MZI) are important components for fiber-optic narrow-band wavelength (de)multiplexing as well as precision sensing. The fundamental concept of the MZI is to split a coherent light, such as a laser, into two different paths. Due to different refraction indices of the two paths, lights processing in the two paths will end up with different phase change or phase displacement. When the lights in the two paths eventually meet together, there occurs the so-called Mach-Zehnder interference.
Please refer to FIG. 1, which is a schematic diagram showing the structure as well as the function of a MZI according to the prior art. The MZI 10 includes an entry 11, a light-splitting portion 12, a first path 13, a second path 14, and a light-coupling portion 15. A single-mode laser in enters the entry 11, and is equally (about 50% each) split into the first and second paths 13, 14. The one enters the first path 13 is denoted as Laser 1, and the one enters the second path 14 is denoted as Laser 2. Since the refraction index of the first path 13 is significantly different from that of the second path 14, the effect due to phase delay to Laser 1 and Laser 2 shall be different. When Laser 1 and Laser 2 meet at the light-coupling portion 15, interference occurs due to the phase difference thereinbetween. Theoretically, the effect of interference that ends up with high extinction ratio is produced if the phase difference between two lights interfering each other is an odd-number time to the mathematical constant pi. Therefore, if the phase difference between the lights proceeding in the two paths 13 and 14 can be effectively controlled, it can be used for controlling or adjusting laser lights, say filtering for example.
According to the prior art set for above, two light paths of a current MZI are disposed at separated locations. To produce sufficient phase difference between the lights in each light path, both the two light paths should have sufficient length in order to provide propagation lengths for the lights. Besides, there are cost related concerns regarding the manufacturing for the light-splitting portion 12 and the light-coupling portion 15. Therefore, if the two light paths can be significantly shortened so the dimension of the MZI can be reduced on one hand, and a more cost-effective method for manufacturing the light-splitting portion and the light-coupling portion is developed on the other hand, breakthroughs can be obtained at two important aspect, which are cost and miniturization, and advantages in terms of application and commercialization are so achieved.
The present invention uses a single optical fiber for manufacturing a MZI apparatus. The novel structure design for the MZI needs only some simply steps to make the light-splitting portion, the light-coupling portion, and the two light paths of appropriate length simultaneously. Compared with the prior art design, the MZI according to present invention has breakthroughs not only in the aspect of cost but also miniturization. Besides, the present invention makes use of the core and cladding of the optical fiber for being the two light paths. Since the disturbing conditions from the environment that the core and the cladding of the optical fiber exist are the same, the stability of the MZI provided by the present invention is less affected by its operation environment.